Coronary artery disease and its related cardiac disorders represent the most common cause of death in the USA and Western world. Although the recent advancements in treatment have led to improvements in patient outcome, the most critical factor is the correct assignment of these therapy options and the precise treatment evaluation thereafter [1, 2]. Myocardial perfusion imaging (MPI) has demonstrated excellent diagnostic accuracy, superb ability to perform risk stratification, and capability for demonstrating therapeutic benefit when applied in the management of the cardiac patient [1-5]. Recently, MPI advanced further with the addition of hybrid PET/CT and SPECT/CT systems that permit the integration of the presence of coronary artery calcium and the degree of coronary artery luminal narrowing with the impairment in myocardial vasodilator function.
PET-CT and MPI Probes
Integrated PET-CT has been widely applied in clinical care as a novel diagnostic imaging tool in patient management owing to its high sensitivity and good resolution. Application of PET-CT has been extended to cardiovascular-related diseases and is undergoing rapid expansion in this area [6-8]. Cardiac PET-CT imaging is advancing the ability to image the structure and function of the heart and vasculature by providing concurrent quantitative information about myocardial perfusion and metabolism with coronary and cardiac anatomy. Precise measurement of regional blood flow has significant clinical importance in identifying ischemia, defining the extent and severity of disease, assessing myocardial viability, establishing the need for medical and surgical intervention, and monitoring the effects of treatments [7, 9]. For myocardial perfusion PET-CT imaging, the positron-emitting radiopharmaceutical must be taken up into the myocardium in proportion to blood flow in order to evaluate areas with reduced blood flow (for example due to ischemia). Several tracers have been used for evaluating myocardial perfusion with PET in clinical practice, including 82Rb chloride, 15O-water, and 13N-ammonia. The short physical half-life of these isotopes allows rapid sequential imaging of rest and stress perfusion. However, the short half-life (from 1-10 min) limits the duration and timing of imaging. Commercial distribution of such agents also is limited, and their associated production costs can be very high. Development of long-lived perfusion imaging agents, labeled with isotopes such as 18F (T½=110 min) or 64Cu (T½=12.7 h), could potentially be distributed by unit doses or with kits, and may allow delayed cardiac imaging following administration. In fact, several 18F labeled tracers are currently being tested in clinical trials for myocardial PET imaging. For example, 18F-flurpiridaz (also known as 18F-BMS747158-02) and 18F-fluorobenzyltriphenylphosphonium (18F-FBnTP) have demonstrated their great potential for PET MPI [10-14]. These promising results clearly demonstrated that PET imaging is valuable tool for the management of the cardiac patient, and novel MPI agents need to be developed to complement currently used 13N—NH3 and 82Rb. Recently, longer-lived PET isotopes, including Cu-64, have been investigated for myocardial perfusion imaging [15, 16]. Agents such as 64/62Cu-pyruvaldehyde-bis(N4-methyl-thiosemicarbazone) ([64/62Cu]-PTSM) have been developed for PET cardiac imaging. However, this tracer has high liver uptake that results in spillover into the inferior wall of myocardial. Moreover, this tracer clears slowly from the blood pool due to the association with serum albumin [16].
Uptake of Triphenylphosphonium (TPP) Cations by Mitochondria
There is a great need to develop novel PET myocardial perfusion imaging agents with optimal imaging property and longer radioactive half-lives than conventional PET MPI agents. Recently, radiolabeled triphenylphosphonium (TPP) ion exhibited optimal characteristics as a PET imaging perfusion tracer due to its significant heart uptake and kinetics [17-20]. The biophysics of the movement of TPP cations across phospholipid bilayers has been extensively studied and is well understood [21, 22]. It has been shown that specific molecular transport methods are not necessary for TPP compounds, which display extensive binding to the matrix surface of the mitochondrial inner membrane [23-25]. A critical parameter affecting the rate and extent of uptake of TPP cations is their hydrophobicity [25]. As the hydrophobicity increases, the activation energy for transport of TPP cations across the plasma membrane is lowered, which will greatly enhances the rate of uptake. In summary, the uptake of TPP cation into the organs from the circulation is driven by the hydrophobicity, and plasma/mitochondrial membrane potentials.
Rationale of 64Cu-TPP Probe Development
With 12.7 h half-life and simple labeling procedures, 64Cu labeled MPI agents could potentially be distributed by unit doses or with kits, and allow delayed cardiac imaging following administration. An ideal 64Cu labeled MPI probe need to have the high heart uptake, proportional to blood flow with little redistribution, while maintaining low liver and lung uptake. Previously, 64Cu-radiopharmaceutical has been developed based on TPP ions. However, those bifunctional chelators (BFCs) generally have negatively charged COOH groups (such as DOTA or NOTA), which would decrease the voltage-dependent uptake of TPP based probes. Moreover, 64Cu-DOTA complexes were found unstable in vivo, which would lead to nonspecific liver uptake [26, 27]. Stable attachment of radioactive 64Cu2+ is therefore another critical factor to consider.